Methods and software for retrospectively gating a set of images

ABSTRACT

Methods, devices, and software for selecting image slices. The image slices are preferably taken during a quiet part of the patient&#39;s heart cycle in which the heart is relatively motionless so as to reduce blurring and the introduction of artifacts into the resultant reconstructed projection image of the heart. The methods of the present invention can analyze information regarding the duration of the patient&#39;s R-R cycles and allow the user to choose from a plurality of selection criteria, typically an absolute time or a percentage of the R-R cycle, so as to select slice images for inclusion in a projection image. The ability to choose from a plurality of selection criteria provides for flexibility to account for irregular heartbeats or a change in the duration of the patient&#39;s R-R cycles. In exemplary embodiments, the resultant projection image can be used for coronary calcium scoring or 3-D rendering of the heart.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims benefit to U.S. Provisional PatentApplication S. No. 60/306,311 filed Jul. 17, 2001, the completedisclosure of which is incorporated herein by reference.

[0002] The present application is also related to U.S. PatentApplication entitled “Graphical User Interfaces and Methods forRetrospectively Gating a Set of Images” filed herewith, (Attorney DocketNo. 021106-000420US) and U.S. Patent Application entitled “Methods andSoftware for Self-Gating a Set of Images,” also filed herewith,(Attorney Docket No. 021106-000430US), the complete disclosures of whichare incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0003] The present invention relates generally to medical imaging. Morespecifically, the present invention relates to gating of an image scanto improve calcium scoring of a patient's heart and coronary arteries.

[0004] CT scanning of the heart is an increasingly common procedure toobtain information about the presence of calcification in the coronaryarteries. Unfortunately, two body motions can interfere with the qualityof the images obtained by the CT scanner: the heart motion and thepatient's breathing motion. A normal heart scan takes about 20 secondsand to reduce the effect of the breathing motion, the patient isgenerally asked to hold their breath to eliminate the breath motion. Theheart motion, on the other hand, cannot be readily eliminated and canlead to blurring, introduction of artifacts into the images, andmisregistration.

[0005] A common procedure to reduce the heart motion is gating. As isdescribed in U.S. Pat. Nos. 6,370,217 B1 and 6,243,437 to Hu et al., themotion of the heart is fastest during systole and relatively motionlessduring diastole. Prospective gating methodologies use anelectrocardiograph signal (ECG) to predict the time of the diastole suchthat the CT scanner can be activated to obtain an image during therelatively motionless diastole period. A major issue with prospectivegating in subjects with irregular heart beats is that the trigger canonly be set to acquire data after the R-wave. If the following beat isshort, the data acquisition may overlap the next systolic period.Retrospective gating, on the other hand, uses the electrocardiographsignal to retrospectively find motionless points in the heart cycle toselect the image slice. In retrospective gating, the ECG signalinformation can be used, in retrospect, to select the slice images thatwere acquired during the diastole. The heart moves through a cycle insomewhat under a second, and a scanners generally take from a quartersecond to a half second to acquire the information for each slice, thusit is possible to select from a number of slices for each cardiac cycle.

[0006] There are two major issues with retrospective gating. The firstis that while reconstruction at finer intervals than the wholeacquisition cycle does not increase the radiation dose to the subject toproduce the extra images, the overlap of the scanned volume and the factthat the scanner's x-ray tube is continuously on (instead of beingturned off during the parts of the cardiac cycle that are not ofinterest) increase the radiation dosage. The second problem is thatgating from an ECG signal requires the placement of electrodes on thesubject and testing to confirm that their placement is adequate. In abusy screening or diagnostic practice the added steps can decreaseutilization and negatively affect the economics of the imagingoperation.

[0007] There are various shortcomings in existing software forretrospective gating. When the operator is performing the selection ofslices, there is no real time feedback as to the adequacy of theselection. Information as to the length of the cardiac cycle during thestudy, convenient ways to ascertain whether it changed during the study,and measurement of any one cycle are also not readily available. Exceptfor manually adjusting each slice (there can be 350-500 slices in astudy), there is no way to account for changes in the cardiac cycle. Allof these contribute to decreasing the certainty with which a particularcoronary calcium score is known, and to increasing the variability ofthe resulting calcium scores.

[0008] Consequently, what is needed are improved methods and softwarefor generating a reconstructed projection image of the patient's heartwhich more fully utilizes the information content of the acquisitioncycle, so that less of the increased dose is wasted or thrown out.Additionally, what is also needed are methods and software that can gatean image scan without the use of an ECG signal.

BRIEF SUMMARY OF THE INVENTION

[0009] The present invention provides methods, software, and computersystems for gating images of the patient's heart.

[0010] In particular, the present invention retrospectively gates a setof image slices by analyzing the duration of the patient's R-R cyclesand selecting a subset of slices based on the duration and change in theduration (if any) of the patient's R-R cycles. Preferably, the chosensubset of images are obtained during a diastole of the patient's heartcycle.

[0011] Exemplary methods of the present invention are used to image apatient's heart. The resultant image from the methods of the presentinvention can be used for calcium scoring, 3-D rendering, or the like.

[0012] Determination of a time point relative to the R-wave is animportant factor for image quality and for reliable diagnostic results.Images acquired during a diastole, in which the heart is fully expandedand relatively motionless, provide the clearest images of the heart.Because the heart cycle of the patient may change during the acquisitionof the images or because the patient may have an irregular heartbeat,the present invention allows for a flexible selection of the time pointduring the R-R cycle in which the image slice is selected.

[0013] In some embodiments, the present invention provides a graphicalillustration of the duration of the R-R cycle over the acquisition timeperiod. The graph can provide the user with a visual indication as tothe presence of any changes in the duration of the R-R cycle.Additionally or alternatively, the present invention can calculate anddisplay numerical information regarding the duration of the R-R cyclefor different periods of the acquisition period. For example, thenumerical information can illustrate the duration and change of durationof the R-R cycle for the first 10 cycles, the middle 10 cycles, and thelast 10 cycles.

[0014] The information regarding the duration of the R-R cycle caninform the user if the patient has an irregular heartbeat or if there isa change in the duration of the R-R cycle over the acquisition period.With this information, the user will be given the option of choosing aselection criteria that is appropriate for the measured R-R cycleduration.

[0015] In exemplary embodiments, the user will be given the option ofselecting from a plurality of selection criteria. One exemplaryselection criteria is an absolute time (before or after) the R-wave.Another exemplary selection criteria is a percentage of the R-R cycle(before or after) the R-wave.

[0016] After the selection criteria has been chosen and applied to theset of image slices, a subset of the image slices will be chosen. Acoronal or sagittal projection image can thereafter by generated usingthe subset of images that were chosen with the selection criteria. Theinvention of this application is not limited to the use of coronal orsagittal projections. Other projections may be chosen, such as those ofthe heart's short or long axis.

[0017] If any of the selected slices are deemed to be not appropriatefor the projection image, the user can scroll through the selected (anddeselected) slices to choose more appropriate slices for inclusion inthe projection image. Advantageously, the user can be provided with animage of the generated projection coronal/sagittal projection imagealongside the projection image that includes all of the overlappingslices of the image set. Moreover, the projection image with theselected slices can be updated in real time so as to show the user theeffect of the added slice(s).

[0018] In one specific embodiment, the present invention provides amethod of retrospectively gating an image scan. The method comprisesacquiring a plurality of overlapping slice images of a volume of tissueof a patient. An ECG signal of the patient is measured and a duration ofeach of the patient's R-R cycles is calculated. The user can choose froma plurality of slice selection criteria based on the duration of thepatient's R cycle and slices are selected from the image scan using theslice selection criteria. Thereafter, a reconstruction of the volume oftissue of the patient is regenerated using the selected slices.

[0019] In another particular embodiment, the method of the presentinvention provides a method of generating an image of a patient's heart.The method includes acquiring a plurality of overlapping slice images ofthe patient's heart. A duration of each cardiac cycle is measured. Sliceimages for inclusion into a reconstruction (e.g., coronal or sagittalprojection) are selected based on the measurement of the duration of thecardiac cycle.

[0020] The duration of the R-R cycle can be analyzed to see if there areany changes in the duration. If the duration of the R-R cycle isrelatively consistent throughout the acquisition period, a selectioncriteria of an absolute time can be used to select the slices. If,however, the duration information shows that there is a change over theacquisition period, the selection criteria of a percentage of the R-Rcycle can be chosen.

[0021] In another embodiment, the present invention provides a method ofselecting slices from an image scan. The method comprises acquiring aplurality of overlapping slice images of a patient and an ECG signal ofthe patient. A first set of slice images from the plurality ofoverlapping slice images are chosen based on the timing relative to theECG signal. A maximum intensity projection algorithm is applied to eachslice of the first set of slices and slices immediately adjacent theslices in the first set to generate a maximum intensity projection slicethat contains, on a pixel by pixel basis, information from a pixel withthe highest intensity in the three slices. A second set of slices isgenerated from the maximum intensity projection slices.

[0022] For a further understanding of the nature and advantages of theinvention, reference should be made to the following description takenin conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023]FIG. 1 schematically illustrates a simplified retrospective gatingmethod of the present invention with the optional steps in dotted lines;

[0024]FIG. 2 illustrates one exemplary graphical user interfacedisplaying information regarding the duration of an R-R cycle of apatient;

[0025]FIG. 3 is an enlarged portion of the R-R cycle information of FIG.2;

[0026]FIG. 4 illustrates a graphical user interface having the view taband view screen displayed and slice images selected during a diastole;

[0027]FIG. 5 illustrates a graphical user interface displaying sliceimages selected during a systole;

[0028]FIG. 6 is a graphical user interface displaying a stretched imageand an overlaid ECG signal;

[0029]FIG. 7 schematically illustrates a simplified method of selfgating a set of image slices;

[0030]FIG. 8 schematically illustrates another simplified method of selfgating as et of image slices with the optional steps in dotted lines;

[0031]FIGS. 9A and 9B are coronal and sagittal projections of apatient's heart, respectively;

[0032]FIG. 10 illustrates a freehand editing of a region of interest;

[0033]FIG. 11 illustrates a straight line editing of a region ofinterest;

[0034]FIG. 12 is an example of an intensity profile;

[0035]FIG. 13 is a smoothed output of a local intensity signal withmarkers indicating maxima obtained from a derivative filter;

[0036]FIG. 14 is an example of a power spectrum;

[0037]FIG. 15 illustrates a graphical user interface in which an ECG hasnot been loaded and a user can self-gate the image scan;

[0038]FIG. 16 is an exemplary data flow diagram of self gating; and

[0039]FIG. 17 is a graphical user interface of a self gating preview.

DETAILED DESCRIPTION OF THE INVENTION

[0040] The present invention provides methods and graphical userinterfaces for self gating and retrospectively gating a set of imageslices (referred to herein as an image scan).

[0041] While the remaining discussion focuses on the gating of an imagescan from a CT scanner for use in coronary calcium measurements, itshould be appreciated that the methods and devices of the presentinvention are not limited to such imaging modalities and uses. Forexample, instead of analyzing the image scan for measuring coronarycalcium, the image scan can be used for 3-D reconstructions of theheart, such as those used for CT angiography, or for heart functionstudies, including dynamic studies.

[0042] In some exemplary embodiments, the present invention uses apatient's measured ECG signals taken during the acquisition of the imagescan to gate the image scan. The ECG signal is a repetitive pattern thatreflects the electrical activity of the patient's heart. An ECG signalhas a plurality of cardiac cycles (sometimes referred to as R-R cycles),with each cardiac cycle beginning with an R-wave (e.g., highestamplitude peak) during a systole period and ending with a relativelymotionless diastole period. Blurring of the images is most likely tooccur when imaging during systole. Consequently, it is preferable to useimage slices taken during the diastole period so as to reduce the amountof artifacts introduced into the reconstruction image(s).

[0043] Unfortunately, the R-R interval can vary through the image scanand the cardiac cycle will not always occur at regular intervals (e.g.,irregular heartbeat). For example, in many imaging sessions, the subjectis asked to hold their breath so as to reduce introduction of artifactsdue to the breathing motion. The patient's holding of their breath,however, may cause a change in the heart cycle. Additionally, patient'swho have irregular heart beats may not be effectively imaged byselecting a specific point in time during the cardiac cycle. While somestudies have proclaimed that it is best to select a particular time withrespect to the R-wave, (some preferring a certain number of millisecondsbefore or after the R-wave), the selection of an absolute time does notallow for compensation for irregular heartbeats or changing timesbetween successive R-waves.

[0044]FIG. 1 schematically illustrates one method 20 of the presentinvention. First, a set of slice images of a volume of tissue of thepatient is obtained and a coronal or sagittal reconstruction of theslice images can be generated. (Step 22). Acquisition of the image scancan be carried out by any conventional or proprietary CT scanner (e.g.,moving, stationary, single detector, multiple detector, helical, and thelike). A helical scan of the heart can include approximately 350-500overlapping images, while a non-overlapping scan usually includes around40-50 slices. It should be appreciated however, that it may be possibleto use magnetic resonance image (MRI) scanners, ultrasound scanners, orother slice imaging devices to obtain the image scan used in the methodsof the present invention.

[0045] The electrical activity of the patient's heart can be measured byattaching one or more electrocardiograph leads to the patient to monitorthe patient's ECG signal during the acquisition of the image scan. Theelectrical activity can be analyzed to derive information regarding theduration of each of the R-R cycles of the ECG signal (Step 24). The R-Rcycle information allows the user to determine if there are anysubstantial variations in the duration of the R-R cycles over theacquisition period of the slice images. Such information allows the userto make appropriate adjustments to their selection of the slice imagesused for generating the coronal/sagittal projection for calcium scoring,or for 3-D rendering.

[0046] The ECG information can be analyzed automatically by software ormanually by the user to determine the duration of each cardiac cycle(illustrated in FIGS. 2 and 3 as “Duration of R-Cycle”). Based on thecalculated R-R cycle information, the user can choose an appropriateglobal selection criteria of choosing the slice images from the imagescan (Step 26). In exemplary embodiments, the selection criteria forchoosing the slice image(s) includes (1) an absolute time period beforeor after the R-wave or (2) a percentage of the cycle (e.g., 65% of theheart cycle) before or after the R-wave. In exemplary embodiments, theuser will be allowed to separately choose the selection criteria (e.g.,percentage or absolute time) and a timing selection (e.g., before orafter the R-wave).

[0047] It should be appreciated however, that while the preferredselection criteria are an absolute time before or after the R-wave or apercentage of the cardiac cycle before or after the R-wave, that otherselection criteria may be used to select the slice images.

[0048] In some embodiments, when the user selects a percentage of thecycle as the selection criteria, the software of the present inventioncan also display a complementary time value that corresponds to theselected percentage. Similarly, if a user chooses an absolute timeperiod, the software of the present invention can display thecorresponding percentage. This significantly decreases operator load.For instance, for most of the heart cycles, the heart rate may be quiteconstant. The operator can set a preferred time, say, 450 msec beforethe R-wave, and the program will show what percentage of the cycle thisis. The operator can then select for a portion of the scan when theheart rate slows near the end, a percentage that is already availablefrom the software. In another case the heart rate may have increased,and the end of the scan may overlap the following R-wave. The operatorcan then select the complementary time after the R-wave to better centerthe selected image.

[0049] In exemplary embodiments, a graphical illustration of theduration of the R-R cycle can be displayed on a user interface toillustrate the duration of the R-R cycle. Advantageously, the graph ofthe R-R cycle will guide the user toward the patient's irregular heartbeats and show if there are any substantial variations in the length ofthe R-R cycle that may effect the selection of the slices.

[0050] One example of a graphical illustration is illustrated in FIG. 3.Graph 92 shows that the duration of the R-R cycle is substantially thesame length during the entire acquisition period. Graph 94 (in dottedlines) shows that the duration of the R-R cycle changes over theacquisition period.

[0051] In the instance in which graph 92 is relatively consistent overtime, the user can apply an absolute time period (before or after theR-wave) to select the slice images for inclusion in the reconstructionprojection image. Since the R-R cycles are substantially the samethroughout the acquisition period, the absolute time period shouldgenerally fit each of the R-R cycles.

[0052] If the duration of the R-R cycle changes over the acquisitionperiod (shown as a dotted line 94), the user would likely use the“percentage of cardiac cycle” (before or after the R-wave) selectioncriteria to select the slices since the absolute time does notcompensate for irregular or changing times of the R-R cycle. While anabsolute time period, (for example 450 msec before the R-wave) may beappropriate for a first portion of the ECG signal, because the R-R cycledecreases over time, the chosen absolute time period would likely beinappropriate for the latter, shorter R-R cycles since the selectedslice would likely overlap over a portion of the high amplitude R-wave.Thus, such a slice would likely introduce artifacts into the resultantprojection image and reduce the accuracy of the calcium scoring of theimage slice.

[0053] Additionally or alternatively, to graphically illustrate theduration of the R-R cycle, the methods of the present invention can alsonumerically display the duration of the R-R cycle for specific intervalsof the acquisition period of the ECG. For example, as illustrated inFIG. 3, the acquisition period may be broken up into a plurality ofintervals. In one exemplary embodiment, the first interval 104 is thefirst 10 cycles of ECG, the second interval 106 is the middle 10 cyclesof the ECG, and the third interval 108 is the last 10 cycles of the ECG.It should be appreciated however, that the ECG can be separate into anynumber of different ECG intervals, and the present invention should notbe limited to the illustrated three intervals.

[0054] By quantitatively providing the average length of the R-R cyclefor the different intervals, the user will be able to accuratelydetermine which selection criteria to employ. For example, if the R-Rcycle duration varies by more than a certain percentage or time length(typically about 70 msec or about 10% of the R-R cycle), the user willlikely want to employ the percentage selection criteria. But if the R-Rcycle duration difference is less than the certain percentage or timelength, the user will likely want to employ the absolute differencecriteria, as described above.

[0055] Alternatively, instead of choosing a global absolute time periodfor all of the cycles, it may be possible to apply a separate selectioncriteria to each of the intervals of the ECG. Thus, if two of theintervals are consistent and the third interval is changing in durationor at a lower duration than the first two intervals, it may bebeneficial to apply an absolute time selection criteria to the first twointervals and a shorter absolute time duration or a percentage of cycleto the third interval. For example, for the illustrated example in FIG.3, as first attempt, the user can select a slice image that is 450 msecbefore the R-wave for first 10 cycles, 450 msec before the R-wave formiddle 10 cycles, and 400 msec before the R-wave for last 10 cycles. Inthis manner, an optimal selection can be achieved, the possibilitiesbeing limited by the acquisition process, and not the gating software.

[0056] After the appropriate selection method is chosen and applied, theselected slices will be combined to generate a correctedcoronal/sagittal projection. (Step 28). In some embodiments a bilinearalgorithm is used to generate the correct aspect ratio coronal/sagittalprojection. It should be appreciated however, that other conventionalinterpolation and scaling algorithms can be used.

[0057] The combination of functionalities and flexibility in choosingthe slices allow for convenient and at the same time highly specificselection of slices on the basis of timing with respect to the ECGsignal. Because the selection of the slices can be displayed to the userin real time (described below), the user can rapidly assess the adequacyof the timing selection of the slice images.

[0058] If the projection images are deemed to be acceptable, theselected slices can be calcium scored or 3-D rendered, if desired. (Step32). Because there is an overlap of the slices during scanning, andbecause the x-ray tube is on during the full cardiac cycle instead ofjust during the acquisition of the desired time interval within thecycle (as in prospective gating) there is an increase in deliveredradiation dose to the patient. Such a dosage increase in unavoidable,but retrospectively it is possible to obtain information from theadditional radiation dose. After acquisition, the reconstructionsoftware can generate additional slices at finer intervals than thosedetermined by table motion and scanner rotation speed, typically tentimes finer. In methods which analyze the slice images for calciumscoring, the calcium will be very bright in the images. Using a maximumintensity projection algorithm, the selected slice and its two immediateneighbors can be analyzed to select the brightest pixel in each of theslices. The slice that has the brightest pixel can then be chosen forinclusion in the calcium scoring study. Thus, the process of the presentinvention effectively utilizes three out of ten images (e.g., the“selected” slice and its two neighbors) instead of just one out of everyten images.

[0059] Because a CT image is obtained from hundreds of individualprojections and processed through back-projection algorithms,inconsistencies in some projections due to heart motion or motion of apoint in the heart that in some way aliases with the acquisition processcan produce a significant artifact even at a time where the heart isrelatively quiescent. Optionally, if the selected slice images chosen bythe above method are not all deemed appropriate because of such aproblem, the user can manually scroll through the selected slice imagesand choose other “non-selected” slice images to replace the undesired“selected” slice images. (Step 30). One method of deselecting slicesfrom the image scan is described below, in relation to one exemplarygraphical user interface of the present invention.

[0060] FIGS. 2-6 illustrate some exemplary graphical user interfaces andmethods for gating an image scan. It should be appreciated however, thatthe graphical user interfaces described and illustrated herein are meantonly to be examples, and should not be used to limit the scope of thepresent invention.

[0061]FIG. 2 schematically illustrates one exemplary graphical userinterface (GUI) 40 of the present invention. GUI 40 is generallydisplayed on a user output device such as a computer monitor. GUIincludes a first screen portion 42 for displaying a selected image, asecond screen portion 44 for displaying an ECG that was taken during theimage scan, and a third screen portion 46 for displaying a coronaland/or a sagittal image projection of the selected slices. Typically,third screen portion 46 will display a first projection image 48 that iscomposed of all of the slices of the image scan and/or a secondprojection image 50 that is composed only of the selected images slices.As will be described in detail below, GUI can further include a fourthscreen portion 52 that can be toggled between a variety of views toallow a user to select and display various functions, menus, andinformation. GUI can also include a menu toolbar 53 so as to allow auser to select and toggle between the different functionalities andplug-ins of the software of the present invention.

[0062] In preferred embodiments the GUI 40 of the present invention cansimultaneously display on a single screen a selected slice image, atleast a portion of the ECG signal, and the sagittal/coronalreconstruction projection image that is composed of the selected slices.Such an interface 40 allows the user to view in real-time, the effectthat the choice or change of image slices has on the quality andresolution of the composite projection image. Thus, if the selectedslices do not improve the quality of the coronal or sagittalreconstruction projection image, the user can de-select the slice(s) toimprove the image quality, and hence improve the calcium scoring or 3-Drendering of the patient's heart.

[0063] As shown in FIG. 2 in exemplary embodiments first screen portion42 can display a selected slice image in window 54 and previous and nextslice images in windows 56, 58, respectively. Slice image window 54 caninclude a header 60 that indicates the slice number, zoom level, and thelike. The adjacent slice image windows 56, 58 can include a header thatindicates “Previous Slice” or “Next Slice.” It should be appreciatedhowever, that a variety of headers can be used to indicate otherinformation, if desired. Image windows 56, 58 can include a scroll bar61 that allows a user to scroll through (review) the slice. In someexemplary embodiments, image windows 56, 58 that display thenon-selected slices are smaller in size than image window 54. It shouldbe appreciated however, that if desired, image windows 56, 58 can be thesame size or larger than image window 54 if desired.

[0064] First screen portion 42 can also include user actuatable buttons62, 63 that allows the user to toggle through the other individual sliceimages of the image scan. If user actuates button 63, the image slicethat was originally displayed in window 58 will be displayed in window54, the image slice that was originally displayed in window 54 will bemoved to image window 56, the image originally displayed in image window56 will not be displayed, and a previously undisplayed slice image willbe shown in window 58. Likewise, if a user actuates button 62, the imageslice that was originally displayed in window 56 will be displayed inwindow 54, the image slice that was originally displayed in window 54will be moved to image window 58, the image originally displayed inimage window 58 will not be displayed, and a previously undisplayedslice image will be displayed in window 56.

[0065] As shown in FIG. 4, if the slice image displayed in window 54 isnot a “selected slice,” first screen portion 42 can include a “SelectSlice” button 64 that allows the user to select a previously“unselected” slice that is displayed in window 54 for inclusion into theprojection image displayed in third screen portion 46. Similarly, if aslice displayed in window 54 is a slice that is already selected orincluded in projection image 50, first portion 42 can include a“Deselect” button 65 that, when actuated, can remove the slice frominclusion in the reconstruction projection image. (FIG. 2)

[0066] If through any of the process described therein, there are gapsin the image data, before saving or calcium scoring the gated image, theuser will be warned of the gaps and asked if the gaps should be filled.If the user chooses to fill the gap, the software can automatically fillthe gap by selecting a slice image that is substantially in the middleof the gap.

[0067] As shown in FIG. 4, in exemplary embodiments, first portion 42can also include a “Previously Selected Slice” button 66 and a “NextSelected Slice” button 68 that allows the user to jump to the next orprevious selected slice in the image scan. In exemplary embodiments, thenext selected slice will be a slice that corresponds to a similar timepoint during the R-R cycle, as described above.

[0068] Windows 48, 50, 54, 56, 58 can be zoomed in and out, panned toadjust the size of the image displayed. The zooming and panning can bedone synchronously for all of the windows, or the zooming of each windowcan be performed independent of each other.

[0069] Referring again to FIG. 2, second screen portion 44 of GUI 40 caninclude an ECG field 70 that displays a patient's ECG signal that wastaken during the imaging of the patient's heart. In most embodiments,only a portion of the entire ECG reading will be displayed on thescreen. Thus, a scroll bar 72 and a zoom bar 74 can allow the user toscroll through the ECG and/or to zoom in and out of the ECG.

[0070] The ECG field can be highlighted, typically through a differencein colors or shading from a background of the ECG field, to indicatewhich slices are chosen relative to the ECG for inclusion into theprojection image 50. For ease of reference, the selected slice imagethat is displayed in window 54 will generally have a different shadingfrom the ECG field background and the highlighting of the other selectedslices. In one exemplary embodiment, the slice displayed in window 54will be identified in the ECG field by a light red band 76, and theother selected slices will be identified by a blue band 78.

[0071] In some embodiments, if the user wishes to manually measure thetime interval of an R-R cycle(s), the user can measure the time intervalbetween two arbitrary or chosen points within the ECG setting oneboundary delimiter by clicking into the ECG and dragging the freeboundary delimiter with a mouse, or other input device, to the secondpoint on the ECG. A field below the ECG can then display the time lengthbetween the two selected points (not shown).

[0072] As seen further in FIGS. 2, 4, and 5, information regarding thenumber of selected slices, position of the current slice in the ECG (inmilliseconds), and the position of the current slice in millimeters, canbe placed below the ECG field to provide information to the user aboutthe selected slice and ECG.

[0073] As the user scrolls through images in the first portion 42, theuser can merely click on the image window 54 to center the ECG cardiaccycle within the ECG field so that the user can simultaneously view theselected image slice and its corresponding cardiac cycle. Alternativelyclicking on a portion of a stretched (or normal) reconstructionprojection will display such a slice in window 54 and center thecorresponding ECG signal in the ECG field. Moreover, the user can usescrollbar 72 below the ECG field to scroll through the R-R cycles untilthe selected R-R cycle is displayed within the ECG field. As noted, theselected R-R cycle will be highlighted a different color from the otherselected R-R cycle slices. Also, clicking on the ECG display will selecta slice with its center closest to the point where the user had placed acursor. The slice will be highlighted on the ECG to display the locationof the slice relative to the ECG.

[0074] Third screen portion 46 can be configured and sized to displayone or more reconstruction projection images. In exemplary embodiments,third screen portion 46 can display a coronal and a sagittal projectionimage of the slices. Alternatively, third screen portion 46 can displayonly a projection image that is composed of only the selected slices. Ifdesired, in order to provide a visual impression of the image quality ofthe projection image with only the selected slices 50, a projectionimage having all of the slice image of the image scan 48 can be shownadjacent image 50. Additionally, the third screen portion may only showthe coronal/sagittal projection image having only the selected slices.

[0075] Third portion 46 can include a line 80 across the reconstructionprojection image to indicate the position of the slice image that isdisplayed in window 54.

[0076] In exemplary embodiments, fourth screen section 52 can be toggledbetween an “ECG” screen 82 (FIGS. 2 and 3) and a “View” screen 84 (FIGS.4 and 5). Once the View tab 83 is activated, a View screen 84 will bedisplayed. View screen 84 includes buttons 86, 88 that allow the user tochange the view of the reconstruction image 50 between a coronal (orMPR3) and a sagittal (or MPR2) projection.

[0077] Fourth screen portion 52 can include an ECG tab 90 which whenclicked or otherwise selected by the user will display ECG screen 82 soas to display information about the average length of the R-cycle forthe patient for certain intervals of the ECG. In some embodiments, theECG screen will have a graph which illustrates the duration of thepatient's R-R cycle. Such a graph can graphically illustrate theduration of the R-R cycles, typically in milliseconds. Thus, if the R-Rcycle is seen to be decreasing or increasing over time, the user canmodify the method in which the slice images are selected.

[0078] For example, as shown in FIGS. 2 and 3, the graph 92 shows thatthe R-R cycle stays relatively constant through 30 measured R-R cycles.For such information datasets, selecting an absolute time before orafter the R-wave will likely be sufficient to select the appropriateslice images for inclusion into the projection reconstruction. If,however, the patient had graph 94, which shows a change in the durationof R-R cycle over time (e.g., a slope in the graph), it would probablybe beneficial to use a percentage of cardiac cycle as the selectioncriteria for the slices.

[0079] Once the user decides on a selection criteria, the user canactivate View tab 83 to bring up View screen 84. View Screen willinclude fields that 96 allow the user to enter their desired selectioncriteria. View Screen 84 can also include an Apply Values button 98 thatapplies the slice selection criteria for the R-R cycles, a Deselect allSlices 100, Center the ECG image in the ECG field 102 and described morefully below.

[0080] If the Deselect All Slices button 100 is activated, the slicesthat were selected for inclusion in the reconstruction projection imagewill all be deselected and the user will be allowed to reselect theslice images for the reconstruction projection, using the sliceselection criteria input into the specified field. Activation of theCenter button 102 will center the ECG cardiac cycle within the ECG fieldfor the image slice that is displayed in window 54.

[0081] As illustrated in FIG. 6, in order to display a stretched imageof the coronal and/or sagittal reconstruction projection, the user canactivate a input box 105 in the fourth screen section. A stretched imageallows examination as to whether a particular slice fits well withrespect to its neighbors, or whether another slice may fit better.

[0082] Referring again to FIGS. 4 and 6, checking of the box willprovide a stretched coronal or sagittal projection of the reconstructionof slices in third screen section 46. Checking of box 105 will make a“Display/Overlay ECG” box 107 active to allow the user to overlay an ECGsignal over the stretched reconstruction projection. If desired, theuser can overlay the ECG over the stretched image so as to allow theuser to determine if a slice fits well with respect to its neighbors ina particular cycle, or whether another slice may fit better.

[0083] The stretched view is needed because the spatial resolution ofthe computer screen/eye combination is not sufficient to adequately viewthe image with the necessary detail. Zooming the image would require toolarge a space on the screen for the in-plane dimension, so that theimage is zoomed only along the slice axis and thus appears stretched.

[0084] When displaying a stretched image with an overlaid ECG, fourthscreen portion 52 can include a “Match” button 113. As shown in FIG. 6,the “Match” function will scale and zoom the stretched view of the ECGin window 109 to match the portion of the ECG displayed in window 111,the two ECGs being displayed synchronized. In addition, with the clickof a button on the input device, the software of the present inventioncan also center the ECG and the stretched view on the current slice, incase it is scrolled out of the field of view.

[0085] If the user desires to replace a slice image from the stretchedview, the user can scroll through the slices displayed in window 42until the highlight marker 76 in the ECG field 70 is over the desiredportion of the cardiac cycle within field 70. Thereafter, the user canactivate the select slice button 64 to include the slice in thestretched view.

[0086] Referring again to FIG. 4, the user can choose to toggle betweena coronal projection and a sagittal projection to alter the view of theprojection image by activating the input field 86, 88. In otherembodiments, it may be possible to activate both of fields 86, 88 so asto simultaneously display the coronal and sagittal projections.

[0087] The method of using the graphical user interfaces of the presentinvention will now be described. The software of the present inventioncan be a stand alone software package or it can be in the form of aplug-in into a software package, such as a calcium scoring package.First, the user can load an image scan, or a collection of slicesacquired during imaging into the software. The image scan can be a savedimage scan, or alternatively, the image scan can come directly from a CTscanner attached to the computer running the software of the presentinvention.

[0088] If available, ECG information that corresponds to the imagedataset can also be downloaded into the software. If an ECG informationis not available, the software can use the self gating methods describedbelow, to gate the images. If an ECG is loaded into the software, theECG will be displayed in ECG field 44 and a composite sagittal/coronalimage of all of the slices of the image scan will be displayed in window46. In some embodiments, a center slice of the image set and its twoneighbors can be displayed in windows 54, 56, and 58. As can be seen inFIG. 2, the composite image with all of the slices will generally have ajagged outline due to the movement of the heart. To improve theselection of the slices included in the sagittal/coronal projection, theuser can click on the ECG tab 90 to display the R-R cycle information.

[0089] After analyzing the R-R cycle information for any changes in theduration of the R-R cycle during the acquisition period, the user canchoose from a plurality of selection criteria, typically either anabsolute time period or percentage of cycle period. The user can selectthe View tab 83 and enter the selected criteria in the appropriate field96. In some embodiments, if the user selects an absolute time selectioncriteria for a slice, the program will automatically calculate acorresponding percentage of cycle that corresponds to the absolute timeentered by the user for that slice (Window 54). Similarly, if the userselects a percentage of cycle as the selection criteria, the softwarewill automatically calculate and display a corresponding absolute timerelative to the R-wave.

[0090] Once the user has entered the selection criteria, the user canactivate the Apply Values button 98 to select the slices for inclusioninto the projection image. As shown in FIG. 2, once the selectioncriteria value is applied, the user will be provided with acoronal/sagittal projection using only the selected slices in window 50that is adjacent the coronal/sagittal projection using all of theselected slices. The ECG will also be highlighted 76, 78 to illustratewhich slices are chosen and the position of the slices relative to theECG.

[0091]FIG. 4 illustrates an coronal image 50 which was selected duringthe diastole. In contrast, FIG. 5 illustrates the coronal projectionimage 50′ that was selected during systole. As can be seen in theimages, the coronal projection image of the heart during systole isnoticeably blurrier.

[0092] If the user desires to re-select the selection criteria, the usercan again click on the View tab 83 and enter a new selection criteria(e.g., a new time or percentage value) until an acceptablecoronal/sagittal projection image is generated. Advantageously, becausethe coronal/sagittal image is updated in real-time when the new slicesare selected, the user can tell, in real-time, the effect of the choiceof the images on the quality of the coronal/sagittal projection image.

[0093] Once the user has found an acceptable “global” selectioncriteria, the user can manually scroll through the slice images toselect or deselect individual slice images of the image scan to improvethe choice of the individual slice images. For example, as shown in FIG.2, to scroll through the selected slices, the user can activate thePrev. Selected Slice button 66 and Next Selected Slice Button 68. Suchbuttons will display in window 54 the Selected slice and in windows 56,58 the slices adjacent the selected slice. If the user wants to keep theslice displayed in window 54, the user can move to the next slice imageby pressing either button 66 or 68. If however, the user wants to selectanother slice, the user can activate the Deselect button 65 and scrollthrough the adjacent slices by activating button 62, 63. Once the userfinds a slice that is acceptable, the user can activate the Selectbutton 64 (FIGS. 4 and 5). The user can repeat this process until all ofthe slices have been selected. Thereafter, the user can save the imagescan (e.g., the selected slices, the sagittal/coronal projection,selection criteria, and the like), and the image of the heart with theselected slices can be calcium scored and/or 3-D rendered. The calciumscoring can be carried out by a separate software program, or it can becarried out by the same program that gated the image scan. Someexemplary computer systems for running the software of the presentinvention, calcium scoring methods, and software are more fullydescribed in co-pending U.S. patent application Ser. No. 10/096,356,filed Mar. 11, 2002 and U.S. patent application Ser. No. 10/126,463,filed Apr. 18, 2002, entitled “Methods & Software for Improving CoronaryCalcium Scoring Consistency,” (Attorney Docket No. 021106-000710US) thecomplete disclosures of which are incorporated herein by reference.

[0094] In another aspect, the present invention provides methods andsoftware for gating an image scan without the use of a gating signal.Because the gating signal (e.g., ECG signal) requires the purchase anduse of additional expensive hardware and software packages, and requiresadded time for placing the electrodes on the patient and confirming theadequacy of the signal being obtained, it is often desirable to be ableto perform an image scan without the use of a gating signal. Exemplaryself-gating methods of the present invention use information derivedfrom the image slices themselves to infer the heart motion without theuse of an ECG signal.

[0095] In one self-gating method, the image slices are selected throughdetection of the size of the heart or pixel intensity in each of theslice images. In another self-gating method, image slices are chosenthrough deriving an average heart rate from the variability of thesignal in the image data and selecting the images based on thecalculated frequency information. In some configurations, a size of theheart is used in conjunction with the frequency measurement to generatethe image of the heart.

[0096] During the quiescent time (e.g., diastole), the heart will beimaged in relatively motionless and fully expanded size. In contrast,when the heart is in systole, the heart will be contracted. By selectingthe images in which the image of the heart volume is largest, the set ofimages will be selected when the heart is in diastole.

[0097] In a first self-gating method illustrated schematically in FIG.7, the software of the present invention selects slice images from theset of slices based on the size of the heart. The first step of thefirst method of self-gating the images is to acquire the set ofoverlapping images of the volume of the patient (Step 192). Selection ofthe images can be done successively by depopulating the slice set (basedon the size of the heart in the image) until the necessary number ofslice images are selected, enough to cover the heart without gaps, whichdepends on slice thickness and heart size. In one exemplary embodiment,depopulating the image scan can be carried out by pairwise comparison.(Step 194). Once the slice images are selected, the coronal/sagittalprojection can be generated and the image of the heart can be calciumscored or 3-D rendered. (Step 196).

[0098] If in some of the methods of this invention there are gaps in theimage data, before saving or calcium scoring the gated image, the userwill be warned of the gaps and asked if the gaps should be filled. Ifthe user chooses to fill the gap, the software can automatically fillthe gap by selecting a slice image that is substantially in the middleof the gap.

[0099] By drawing on a sagittal or coronal view a region of interest(ROI) encompassing one side of the heart, one can determine the state ofthe heart muscle by noting the total signal along the line representingthe slice, or noting how many pixels have the signal of muscle ratherthan the much lower signal of fat of the lung. When comparing a slice toits immediate neighbors, the slice with the most expansion will providea line with a higher total signal, or with more pixels above a specifiedthreshold, than a slice belonging to a point in time with lessexpansion. For pairwise comparison, each slice is compared to oneneighbor, and the one with most expansion kept. This process can stopwhen a gap would be generated by further depopulation of the slices.

[0100]FIG. 8 schematically illustrates another simplified method ofself-gating in which the images are selected by finding the fundamentalfrequency of the heart from the images themselves. The exemplary method200 comprise obtaining a set of overlapping slice images of a volume ofthe patient, typically of the patient's heart. (Step 202). As describedabove in relation to the retrospective gating, the set of images can beobtained with a CT scanner, or an equivalent imaging technology.

[0101] The set of images can be run through an algorithm to generate acoronal/sagittal projection of the volume of tissue of the patient.(Step 204). FIGS. 9A and 9B illustrate a coronal and sagittal projectionof an image of a patient's heart. The images have jagged edges due tothe motion of the heart.

[0102] The user can then highlight one or more region of the heart inthe coronal/sagittal projection (Step 206). Generally, the user canselect some region around the jagged outline of the heart. Moredistinctive outlines around the heart will give better results. Inexemplary embodiments, the regions of the heart can be marked with afreehand region (FIG. 10), a straight line region (FIG. 11). Due to thescanner rotation time, the outline of the heart is generally onlyselected on one side of the heart-outline. It should be appreciatedhowever, that other conventional marking methodologies can be used tomark a region of the image, including the use of automatedboundary-finding algorithms.

[0103] A pixel intensity signal of the images can be generated bysumming the pixel intensities (HU) within the selected region for eachslice in a direction that is perpendicular to the slice direction. (Step208). The result of the summation is a signal graph, as illustrated inFIG. 12. The signal graph will produce a plurality of maximas andminimas, wherein each of the maximas generally corresponds to a maximuminflation of the heart. The position along the horizontal axiscorresponds to the slice number. The intensity is 0 for slices notincluded in the selection. The intensities can be corresponded to theslices, each of which is acquired at a certain point in time, usuallyevery 100 ms. It should be appreciated however, that the point in timein which the image acquisition is performed will vary depending on thepatient's heart rate or other factors, and such parameters can be setaccordingly.

[0104] The signal can be analyzed to extract the time information fromthe images. (Step 210). Extracting the time information from the signalcan be carried out through an analysis of the frequency spectrum and/orthrough analysis of local intensities of the signal.

[0105] In one exemplary method of extracting the time information, aFourier transformation is applied to analyze the frequency components ofthe signal. In the Fourier transformation, the amplitude profile isviewed as a function of frequency. Each function can be representedthrough its Fourier components—by combining a number of sine and cosinefunctions of different frequencies. The signal intensity profile of theslices will provide a repetitive maxima and minima. The sinusoid (e.g.sine or cosine function) of the same frequency as the repetitive patternwill have a large contribution. The goal is to find this principalcomponent, the sinusoid of the corresponding frequency.

[0106] The result of the Fourier analysis will be a series of complexnumbers. Each number corresponds to a sinusoid of a certain frequency.The formula is:${F(k)} = {\frac{1}{M}{\sum\limits_{m = 0}^{M - 1}{{f(m)}^{\frac{{- 2}\pi \quad {imk}}{M}}}}}$

[0107] where m is the slice number, M is the total number of slices andk is the coordinate in frequency space (or k-space) and k/M is thefrequency which corresponds to value F(k),

[0108] From the Fourier analysis, information about the magnitude andphase of the sinusoid can be obtained. The magnitude indicates thestrength of any one frequency component, including the principalcomponent. For each component there is corresponding phase informationwhich contains information about where that component begins. While thephase can theoretically be obtained from this phase information, inpractice, the phase is changing very fast as a function of frequency,and the measurement is not reliable.

[0109] To find the frequency which is the most dominant portion of thefunction, only the information about the “energy” for each frequencycomponent is necessary. The energy of the sinusoid can be read from thepower spectrum, in which power is defined by:

Power(k)=Re(F(k))² +Im(F(k))²

[0110] where Re is the real part of the complex number F(k) and Im isthe imaginary part of the complex number F(k). The result will be asequence which contains only real values. One example of a powerspectrum is illustrated in FIG. 14.

[0111] After the power spectrum is computed, the Fourier series can besmoothed with a Gaussian filter to reduce spurious peaks. Because thetask of finding the heart beat is circumscribed by physiologicrestrictions, the present invention can restrict the search for themaximum frequency to a range of approximately {fraction (1/2000)} ms and{fraction (1/500)} ms, which corresponds to an interval of 500 ms to 2seconds between two heart beats.

[0112] Thereafter, the absolute maximum value in the power series andfrequency can be determined. Additionally, the lower and higherfrequencies next to the maximum frequency where the value is half of themaximum value can be measured (noted as the half-height interval in FIG.14). If the maximum frequency and the half-height interval are found,the frequency which is directly in the middle of the interval defined bythe half-heights is used as the “maximum.” If, however, the half-heightinterval can not be determined, the absolute maximum can be used as the“maximum.” From the maximum frequency, the fundamental frequency (e.g.,the heart beat) of the heart can be determined.

[0113] From the Fourier transformation, the software can determine thefundamental frequency of the heart and generate images of the heart indifferent phases of the heart cycle. As will be described below, theuser can display a plurality of projection images of the heart, in whicheach of the images corresponds to a different phase of the heart cycle.

[0114] Because it is difficult to extract the phase information presentin the Fourier spectrum, the Fourier transformation does not inform theuser as to which slices represent the diastolic phase, systolic phase,and the like. Moreover, such a transformation does not account forirregular heartbeats or a changing of the heartbeat over the imageacquisition period. In order to determine which slices correspond to thediastole, the software of the present invention can analyze the sliceimages to find the biggest heart volume image (e.g., the diastole) inwhich the heart motion is the least.

[0115] To determine the phase of each of the slices, (e.g., to determinewhich slices correspond to diastole), a local intensity signal of theslice images can be run through a derivative filter to produce a graphsuch as FIG. 13. Generally, this method can be used in conjunction withthe results from Fourier analysis, as described above, to find the sizeof the heart in each of the slice images. With the frequency derivedfrom Fourier analysis and phase from the local maxima, slice selectioncan be extended beyond the ROI of Step 206. It should be appreciatedhowever, that it may be possible to use the local intensity profile asan independent algorithm. In such embodiments, the user would need tocover all slices with the selected region of Step 206.

[0116] In such an analysis, as illustrated in FIG. 13 each local peak220 in the intensity signal corresponds to the maximum inflation of theheart. The peaks can be located through a differential analysis with thedifferential filter in which each peak (i.e., local maximum) has a firstderivative of zero and a second derivative that produces a zero crossingresponse.

[0117] From the filtered data, the zero-crossings can be located. Acrossing from a negative number to a positive and back to a negativecorresponds to a maximum. Crossing from a positive to a negative andback to a positive corresponds to a minimum. It should be appreciatedhowever, that the signs of the zero-crossings are dependent on the signof the second derivative filter, which as described above was fixed tobe negative-positive-negative. From the zero crossing intervals, thelocation of the maximum intensity values are found and the slices inwhich the heart is in diastole are chosen.

[0118] Post-processing of the maxima found above can proceed in severalpasses over the slice selection. As an initial step, the distancebetween two adjacent selected slices will be checked to determine if theslices are too close together. In one configuration, the slices will bedeemed to be too close if they are within one third of the heart-ratefrequency found by the Fourier transformation, this being a reasonablelimit for how much the heart rate may change during the study. It shouldbe appreciated however, that in other configurations, a smaller orlarger frequency distance can be used. If the slices are deemed to betoo close, the slice that has the lower intensity value will be removedfrom the image set of selected slices.

[0119] Next, for each selected slice, the algorithm can resample theimages to verify that at least two of the slices' four neighbors arewithin 30% of the heart rate measured by the Fourier analysis so as toavoid irregular spacing. If the slice is outside of the 30% range, theslice will be deleted from the set of selected slices. It should beappreciated however, that it may be possible to use a criteria differentcriteria (i.e., smaller or larger than 30% of the heart rate), ifdesired.

[0120] Thereafter, the algorithm can resample the images to check thespacing between the remaining slices to see if there are any gaps thatare bigger than the slice thickness (which is combination of thethickness of the slice for a stationary scan and the broadeningintroduced by the travel of the patient bed during the helical scan). Ifthere is such a gap, the gap can be filled in with a slice of maximumintensity in FIG. 13 in the location of the gap. It should be noted,that it is preferable to have the slices be spaced so as not to leavegaps not covered by the slice thickness, as noted above. If there areany slices between two slices that are within the heart rate found bythe Fourier analysis, the slices are deleted from the set of selectedslices.

[0121] Generally, the derivative filter algorithm will only cover theselected region of the scan that was marked by the user. Thus, if theuser did not select the entire image additional slices need to beselected. If slices need to be added, a pseudo-selection of slices canbe generated on each end of the selection region. The generated sliceswill be spaced by the frequency found by the Fourier analysis. Across-correlation at various offsets can be performed to obtain the bestestimate of the phase for extension of the frequency information. Theoffset that returns the biggest correlation value is used to extend thedataset to complete the image. The same cross-correlation algorithm canbe applied to the pseudo selection slices, as described above.

[0122] The computed heart rate can then be used to generate multipleslice subsets from the original set of slice images, in which each ofthe slice subsets correspond to a different phase of the heart cycle.(Step 212). The present invention can use software to efficiently selectas many sets as there are redundancy, and present them to the user forselection. Multiple selections can be generated from the frequency butat different phases of the cardiac cycle to give the user the choice toselect one. There are (1/frequency*1/time between slices) differentoffsets from the first slice in the original image set. The softwareprogram selects the i^(th) slice as offset+i*(1/frequency*1/time betweenslices) so as to result in 1/frequency*1/time between slices subsets ofthe original scan. The user can choose the desired set from these.

[0123] Having the fundamental heart rate, however, is not sufficient forthe best selection of slices since the fundamental heart rate does notexplicitly define which slices correspond to the diastolic phase. Thus,to select the images that were obtained during diastole, the heartfrequency information can be used along with the information obtainedwith the derivative filters to obtain time and phase information togenerate an image in which the heart is at its largest volume (e.g.,diastole). (Step 214).

[0124] Additionally or alternatively, the plurality of images of theheart can be ranked by applying a quality measure so as to rank theimages based on heart size. (Step 216). One quality measure algorithmcomprises summing all of the pixel intensity values over a certainthreshold value. The intensity value is normalized by the total numberof pixels in the image to provide the average intensity value of theimage. Thereafter, each of the average intensity values of each of theimages are compared to rank the images relative to each other.

[0125] Another quality measure algorithm counts the number of pixelsabove a threshold value. The number of pixels above the threshold isnormalized by the number of pixels in the image to provide a fraction.The fraction can identify the percentage of the image that the heartoccupies in the image. Generally, the higher the fraction, the betterthe selection. Thereafter, the fractions of each of the generated imagesare compared and ranked relative to each other. It should be appreciatedhowever, that other quality measure algorithms can be used to rank theimages of the heart.

[0126] Thereafter, the images of the heart can be displayed on acomputer output display in order of rank so as to allow the user toselect the phase most appropriate for the scoring of each vessel withinthe heart. (Step 218). Alternatively, it may be possible for thesoftware to automatically display only the image with the highest rank.

[0127] In some methods, the software of the present invention can beused to auto correlate between image pairs and computes the quality ofthe correlation. Times of slow motion produce better correlations thanwhen the motion is rapid. A repetitive pattern can be established fromwhich the quiescent times are selected to create the gated image set.Advantageously, the same graphical user interface of FIG. 2 can be usedto gate the image scan.

[0128]FIG. 15 illustrates a graphical user interface that can be used toself-gate the set of image slices without the use of an ECG signal. Asshown in FIG. 15, menu toolbar 53 can include additional buttons “Edit,”“Clear Selection,” and “Self Gate” that allows the user to self gate theimage scan. The software allows the user to delineate the regions of theheart where the heart motion can be visually observed.

[0129] With the “Edit” button 110, the user can enter an editing/drawingmode in which the user can draw a boundary around a region of the imageof the heart and mark it. The region can be selected by at least twodifferent manners. A first manner is through a straight line selection,in which the user selects a first end point of a straight line and asecond end point of the line to define the region.

[0130] In selecting the region, the region must have a minimum lengthacross the slice direction and a maximum length within the direction ofone slice. If the selected region is too small to obtain enoughinformation for analysis (e.g., less than about three seconds) or toowide so that the signal is lost because of scanner rotation (e.g., morethan approximately half of the image width), the software of the presentinvention can provide the user with an error message to prompt the userto select a different region and to prevent the computation of aheart-rate from unsuitable data.

[0131] In exemplary embodiments, a left click of the mouse defines thefirst endpoint, and a right click of the mouse defines the second point.The line drawn by the user will be used to define a diagonal of arectangular region. In a second manner, the user can use a freehandselection, which allows the user to select a region of arbitrary shape.In one embodiment, the user can depress a “Control” key on a keyboard ofa computer system and move the mouse to draw the region of interest intothe arbitrary shape. Releasing the control key closes the region. Itshould be appreciated however, that the above methods of drawing theregion are merely examples, and other conventional methods ofdrawing/selecting the region can be used.

[0132] The region can be a portion of of the border of the patient'sheart. Advantageously, drawing the region around multiple portions ofthe border of the heart allows the user to see and track differentiallythe motion of the heart through the different portions of the heartcycle. Thus, the user can view the different chambers of the heart as itmoves through the R-R cycle.

[0133] The “Clear Selection” button 112 can delete a region that waspreviously marked by the user. The “Self Gate” button 114 starts theself gating procedure that is described herein.

[0134] Referring again to FIGS. 10-12, to self gate an image scan, theuser marks selected section(s) in the sagittal view or the coronal viewon one side of the heart where the motion can be seen. Motion of theheart will be shown by the jagged edge of the heart in the sagittalimage and coronal image. Straight line boundaries 118 (FIG. 11) orfreehand boundaries 120 (FIG. 10) can be drawn on one or more portionsof an edge of the heart. If desired, the user can select multipleregions.

[0135] Once the regions have been selected, the user can click on theSelf Gate button 114. The self gate software can them compute theaverage frequency of the heart beat using the information of theselected region and generate a number of selection. FIG. 16schematically illustrates an exemplary data flow of the presentinvention.

[0136]FIG. 17 shows one preview screen graphical interface fordisplaying the multiple selections. One selection represents the biggestheart volume based on the frequency information of the selected regionand the intensity values in the marked regions. The other selections arebased only on the frequency information. Each selection corresponds to adifferent phase of the measured heart frequency.

[0137] As shown in FIG. 17, the preview screen 129 includes a mainpreview window 130 having the current selection. The default selectionis derived from the selection that includes the intensity and frequencyinformation. A smaller image 132 of the current selection can also bedisplayed alongside the right portion of the graphical user interface inone of the small preview windows and can be framed by colored frame 134.Clicking or otherwise selecting on another image alongside the smallpreview windows (e.g., the right portion of the graphical userinterface) will display the selected image on the main preview window.The topmost image 132 shows the selection inferred by intensity andfrequency information derived from the image scan. The remaining imagesshow selections that use only the computed heart frequency at differentoffsets (e.g., different phases of the heart's motion). As seen in FIG.17, many of the images at the different phase of the heart hasnoticeable blurring due to the motion of the heart. Nonetheless,providing a plurality of images of the heart in different phases allowsthe user to visually determine which heart image is best.

[0138] The user can select different projection of the current previewby activating the Axial button 136, Coronal button 138, or Sagittalbutton 140. The slider 142 can be activated by the user to scrollthrough the slice projections, if desired. When the user finds aprojection image that is acceptable, the user can click on the OK button144, which will apply the current selection and return to the mainscreen of the graphical user interface (FIG. 2).

[0139] As described above, once the selected image set is deemedacceptable, the image set can be calcium scored and saved. Before savingthe slices as a new DICOM series, the selection of images can be checkedfor gaps. If there are gaps, the number of gaps can be reported to theuser with their size range. The user can then select to ignore the gapsor can elect to fill in the gaps that are bigger than a specifiedthreshold, which the user can specify.

[0140] One method of filling in the gaps is with a slice closest to themiddle point between the selected slices. Of course, these gaps may alsobe filled through low order interpolation algorithms such as nearestneighbor, and in increasing order, linear, cubic and so on, or Fourierinterpolation.

[0141] In another aspect, the present invention provides improvedmethods and software for calcium scoring the images. Retrospectivegating often causes mismatches between the scanner rotation and theheart rate. Consequently, the selected images may not always be equallyspaced such that there are gaps between the images. Most calcium scoringalgorithms, however, are based on algorithms that require a fixedspacing between the slice images.

[0142] Unfortunately, conventional linear or other low orderinterpolation schemes that can be used to generate equally spaced sliceimages from the selected merely blur the images, which degrades thecalcium scoring of the images. The present invention provides a FourierInterpolation that can rescale the dimensions of the image slices thatdoes not introduce blurring or degrade the resolution. A more completedescription of Fourier Interpolation can be found in U.S. Pat. Nos.4,908,573 and 5,036,281 and in Kramer D. M., Li A, Simovsky I, HawryszkoC, Hale J and Kaufman L., “Applications of Voxel Shifting in MagneticResonance Imaging,” Invest Radiol 25:1305, 1990, the completedisclosures of which are incorporated herein by reference.

[0143] While all the above is a complete description of the preferredembodiments of the inventions, various alternatives, modifications, andequivalents may be used. Although the foregoing invention has beendescribed in detail for purposes of clarity of understanding, it will beobvious that certain modifications may be practiced within the scope ofthe appended claims.

What is claimed is:
 1. A method of retrospectively gating an image scan,the method comprising: acquiring a plurality of overlapping slice imagesof a volume of tissue of a patient; measuring an ECG signal of thepatient during the acquiring of the plurality of overlapping sliceimages; calculating from the ECG signal a duration of each of thepatient's R-R cycles; choosing from a plurality of slice selectioncriteria based on the calculated duration of the patient's R-R cycle;selecting slices from the image scan using the chosen slice selectioncriteria; and generating a reconstruction of the volume of tissue of thepatient using selected slices that meet the slice selection criteria. 2.The method of claim 1 wherein the volume of tissue comprises thepatient's heart.
 3. The method of claim 2 comprising calcium scoring theselected slices of the patient's heart.
 4. The method of claim 3 whereinselecting further comprises employing a maximum intensity projectionalgorithm to analyze the selected slices and their adjacent slices togenerate the image set of slices for calcium scoring.
 5. The method ofclaim 1 wherein the selection criteria is chosen from at least a choiceof: an absolute time period before or after an R-wave of the R-R cycle;or a percentage of a R-R cycle before or after the R-wave.
 6. The methodof claim 1 comprising filling in gaps between the selected slices. 7.The method of claim 1 comprising displaying the duration of the R-Rcycles to a user on a display device.
 8. The method of claim 1comprising displaying a numerical indication of the duration of the R-Rcycle.
 9. The method of claim 8 comprising segmenting and displaying theduration of the numerical indication of the duration of the R-R cyclesinto a first portion, a middle portion and an end portion.
 10. Themethod of claim 1 comprising displaying a graph of the duration of thepatient's R-R cycles over a period of acquiring the slice images. 11.The method of claim 1 comprising deselecting at least one of theselected slices for at least one of the cardiac cycles of the ECG andchoosing a replacement slice.
 12. The method of claim 1 whereinacquiring is carried out with a CT scanner.
 13. The method of claim 1wherein the reconstruction of the volume of tissue is a coronal orsagittal projection of the patient's heart.
 14. The method of claim 1comprising displaying a reconstruction of the volume of tissue which hasall of the slice images of the image scan adjacent to the reconstructionof the volume of tissue with the selected slices to illustrate imagequality of the reconstruction of the volume of tissue with the selectedslices.
 15. The method of claim 1 comprising displaying at least oneselected slice simultaneously with the reconstruction of the volume oftissue with the selected slices.
 16. The method of claim 1 comprisingdisplaying a selected slice and slices that are adjacent the selectedslice.
 17. The method of claim 1 comprising: displaying at least aportion of the ECG signal; and highlighting portions of the ECG signalthat correspond to the selected slices.
 18. A method of generating animage of a patient's heart, the method comprising: acquiring a pluralityof slice images of the patient's heart; measuring a duration of eachcardiac cycle of the patient's heart during the acquiring of theplurality of slice images; and selecting slice images to cover thevolume of the patient's heart based on the measurement of the durationof the cardiac cycle.
 19. The method of claim 18 wherein if the durationof the cardiac cycle is substantially consistent, selecting comprisesselecting an absolute time before or after an R-wave of each of thecardiac cycle.
 20. The method of claim 18 wherein if the duration of thecardiac cycle changes over time, selecting comprises selecting apercentage of the cardiac cycle before or after an R-wave of the cardiaccycle.
 21. The method of claim 18 wherein selecting comprisesselectively choosing an absolute time or a percentage of the cardiaccycle before or after an R-wave of the cardiac cycle.
 22. The method ofclaim 18 comprising calcium scoring the selected slice images of thepatient's heart.
 23. The method of claim 18 comprising 3-D rendering thepatient's heart.
 24. A method of selecting slices from an image scan,the method comprising: acquiring a plurality of overlapping slice imagesof a patient with a CT scanner; acquiring an ECG signal of the patientduring the acquiring of plurality of overlapping slice images; choosinga first set of slice images from the plurality of overlapping sliceimages based on a timing of the slice image relative to the ECG signal;applying a maximum intensity projection algorithm to each slice image ofthe first set of slices and slices immediately adjacent the slice imagesin the first set to generate a maximum intensity projection slice imagethat contains, on a pixel by pixel basis, information from a pixel withthe highest intensity in the three slice images; and generating a secondset of slice images from the maximum intensity projection slices. 25.The method of claim 24 comprising generating a coronal or sagittalreconstruction with the second set of slice images.
 26. The method ofclaim 24 wherein the ECG signal comprises a plurality of cardiac cycles,wherein each of the cardiac cycles includes an R-wave, wherein choosingcomprises selecting slices that are at an absolute time period before orafter the R-wave.
 27. The method of claim 24, wherein the ECG signalcomprises a plurality of cardiac cycles, wherein each of the cardiaccycles includes an R-wave, wherein choosing comprises selecting slicesthat are a percentage of the cardiac cycle before or after the R-wave.28. The method of claim 24 comprising calcium scoring the second set ofslices.
 29. The method of claim 24 comprising 3-D rendering the secondset of slices.
 30. A method of retrospectively gating an image scan of apatient's heart, the method comprising: acquiring a plurality ofoverlapping slice images of the patient's heart; measuring an ECG signalof the patient during the acquiring of the plurality of overlappingslice images; calculating from the ECG signal a duration of each of thepatient's R-R cycles; choosing from a plurality of slice selectioncriteria based on the calculated duration of the patient's R-R cycles,wherein the selection criteria is chosen from: an absolute time periodbefore or after an R-wave of the R-R cycle; and a percentage of theduration of the R-R cycle before or after the R-wave; selecting a subsetof slice images from the overlapping slices of the patient's heart usinga chosen slice selection criteria; generating a coronal or sagittalreconstruction of the patient's heart with the selected subset of sliceimages; and performing at least one of a calcium scoring and 3-Drendering of the subset of selected slice images.